Sound-producer.



R. A.,FESSENDEN. sounn PRODUCER.

APPLICATION FILED FEB. 28.19%!- Patented Sept. 3, 1918.

UNITED STATES "PATENT OFFICE.

aEtfiNAI-D A. FESSENDEN, or BROOKLINE, MAssAcH'usE'r'rs, AssiGNOn. T0SUBMABINE SIGNAL COMPANY, or PORTLA D, MAINE, A CORPORATION or MAINE.

souNnraon ucaa'.

To all whom it may concern:

Be it known that I, REGINALD A. FEssEN-. DEN, of Brookline; in thecounty of Norfolk and State of Massachusetts, a citizen of the UnitedStates, have invented a new and'useful Improvement in Sound-Producers,of

which the following is a specification.

My improvement has for its object theincrease in the efficiency of soundproduction, and more particularly of diaphragm sources of sound, thoughnot limited to this type of source.

In 1905 applicant overcame the difliculties which had hitherto preventedthe measurement of the comparative sensitiveness of the ear to sounds ofdifferent frequencies by splitting up the ultra-audible impulsesproduced by his high frequency generator by means of a commutator, andusing the groups noW found that the frequency for which the of Wavetrains of different frequencies so produced to generate sounds.

The curve so obtained, by plotting the sound frequencies against auralsensibility sliowecla marked increase at 500 frequency and a maximum atapproximately 920 frequency. A patent for the application of thisdiscovery was granted to applicant, 2'. e., U.

S. Patent No; 918,306, dated April 13, 1909.

As the result of experiments of locating direction by sound shadows,applicant has ear is most sensitive, thatis, 920, is that which is mostefiicient in giving good allaround reception of sound, combined withgood efficiency of sound source location of sound waves impinging onround bodies of the size of the human head.

' ble or other frequencies, difficulties have been met with. Forexample, it has been proposed to produceisound for submarine signalingand other purposes by means of small diaphragms moving through largeampli-' tudes instead of large diaphragms' moving through smallamplitudes.

Applicant, by developing the mathematical theory of sound along newlines, and confirming the mathematical results by ex- Specification ofLetters rum. P t t t, 3, 9

Application filed February 28, 1917. Serial NO. 151,611.

'. periments, has discovered that the product,

of area and amplitude is no criterion of sound production efliciency,but that to produce sound efficiently, the radius of the diaphragm mustbe of a proximately the same length as or longer t han the quarter wavelength of sounds of the frequency generated, in the medium in. which thediaphragm is immersed.

For example, a telephone receiver diaphragm should have a diameter ofapproximately seven and a half inches or more, and I .a diaphragm forproducing a fog signal of afrequency of 36 should, applicant hasdiscovered, he 15 feet or more in diameter, for est efliciency.

In putting this discovery into practice, and in using these new types ofdiaphragms, with radii approximately equal or greater in size thanquarter wave length, applicant has met with certain dilficulties, meansfor overcoming which are also herein described and claimed.

For example, if a wooden diaphragm of say two inches thickness be usedtoproduce a sound in Water of frequency 440, and the diameter of thediaphragm be five feet and the vibrating force be applied at the centerof the diaphragm, the elastic wave in the wood will travel out from thecenter to the edge of the diaphragm in the wood so much slower thanitwill through the water, owing to the flexural wave in a wooden structureof such dimensions-being much slower than the compressional wave inwater, that very little sound will be produced. And even ifthe diaphragmbe made a foot thick still the result will be small.

Similarly, if asteel diaphragm one thirtysecond of an inch thick and'ten feet diameter be used to generate sound of frequency 1000, theresult will be very inefficient for the same reasons. i

In this latter case the diflicul'ty may be overcome by using thickersteel, as has been the custom to date, or using a built-up girder. But Iprefer ,to accomplish the result in another way, by driving thesoundemitting surface from a number of points, as shown in the figures.

In the drawings accompanying this specification, Figures 1, 2 and 3;show semidiagrammatically, apparatus suitable. for use in connectionwith this invention.

Fig. 1 shows the diaphragm 11, made of 1/32 steel, and 10 feet diameter.12, 12,

12, 12', are piano wires, electricall 'welded at one end to -thediaphragm, a out six inches apart, the other ends being attached to theoscillator 13. The difference in length between the wires 12,. 12,and12, 12 being less than' 2 feet, and the longitudinally elastic Wavesalong-the wires due to the vibration of the oscillator traveling at avelocity of 17,000 feet per second (velocity of sound in steel) thedifference in time between the arrival of vibrations at the center andedge of the diaphragm will be only 1/10,000 of a second, and thediaphragm will work as a whole efliciently.

In place of using a continuous sheet of metal as a diaphragm, a numberof dia phragms may be .used, inserted in holes in .a rigid surface. Buthere another element comes in, i. 6., whether the separate diaphragmsaredrivenin phase or out of phase,

and the distance between th'e-diaphr agms must be adjusted accordingly;

' Applicanthas determined mathematically, and confirmed 'it byexperiment, that if the diaphragms are driven in phase, theircentersshbuId be of the order one half however, their centers shouldbeof the order wave length or less apart. Fig. 2 shows such anarrangement, the diaphragms 14,

14, '14, 14, being driven in this case by' small rods 15, 15', 15 15,attached at one end to the diaphragms:14,{14, 14, 14, andat the other tothe crank shaft 16, driven by the motor 17. V

If the diaphragms are not driven in phase,

of one half Wave length or more apart, as

' shown in Fig. 3, where the diaphragms 18,

18, 18, 18, are/driven by the telephone receiver ma ets 19, 19, 19, 19,the'electro magnet coil of each receiver being attached to an individualmicrophone button, as

shown, and the buttons 20, 20, 20, 20, being mounted on the large woodendiaphragm- 21. Batteries 22, 22, 22, 22, are used in the circuits of thereceiver magnets and buttons, as shown.-

' On -the wooden diaphragm 21 7 being talked against, the-microphonebuttons sum of the diaphragms.v

20, 20, 20, are vibrated and the diaphragnis 18, 18, 18, 18, reproducethe speaking. But I have experimentally discovered that the currents inthe circuits of the buttons 20, 20, 20, 20 are out of phase with eachother, and that this is the'reason. why multiple button transmittershave always heretofore emit-ted: This leads to the peculiar result thatwhile the sound will not be much louder close up to the diaphragms thana single diaphragm, at a distance the sound will be if anything louderthan that due to the This has been verified by experiment. '5 or What Iclaim as my invention is 1. In apparatus for producing sound, adiaphragm having its radius not substantially less than the quarter wavelength of the sound which it is more especially desired to produce. 2.In an apparatus 'for producing sound, a

diaphragm having its radius not substantially less than 'the quarterwave length of the sound which it is'more especially desired to produce,and vibrated by a plurality of vibrating members attached to thediaphragm. 7

3. In an apparatus for producing-sounds,

' a diaphragmhaving its radius not substantially less than the quarterwave length of the sound which .it is especially desired to produce, andcomposed of a number of diaphragm. elements, said elements having thedistance between their centers adjusted in accordance with the degree ofsynchronism between the elements.

REGINALD A. FESSENDEN.

